How do you structure short story work?

There is a sample compiled doc in the Scrivener Short Story Template. At bottom in that doc is a note

I’m wondering how current that information is. It recommends use of underline for italics, whereas some now recommend use of italics. Also, is British practice the same as American, in terms of em dash usage? I always put a space on either side of the em
dash – like this – instead of the way they recommend–like this–but I’m wondering which is right for the US?

I would recommend always checking the submission guidelines for the specific market you have in mind.

As for the em dash, if your work will be typeset, it won’t really matter which way you do it. A typesetter will run an algorithm that corrects for common user practices (e.g., double-spacing at end of sentences), and puts into effect certain conventions. In particular, those em dashes will in all likelihood be typeset flanked by what they call “thin space” – a kind of space character you cannot type on your keyboard.

So, the only thing left to consider is whether anyone who is vetting your manuscript cares if you do it this way or that.